This doctrine of Aristotle is interesting because modern science, calling to its aid all the multitudinous inventions that ingenuity can devise, has reached the conclusion that heat is a condition of motion of the particles of material bodies. Yet the resemblance between this result and the speculation of the old philosopher, though noticeable, is merely superficial, and no certain progress was made in the study of heat till philosophers learned to submit their guesses to the test of experiment.

The progress of science is not a steady advance, there are continual haltings by the way, and even temporary retreats; a long period of stagnation may precede some brilliant discovery or powerful and far-reaching generalization that will at once rouse investigation and usher in a period of great progress; this was true in a marked degree of the study of light.

Early speculation taught that light was an emanation thrown out in straight lines from the luminous body. But during the seventeenth century the theory that light consisted of waves or undulations coming from the heated body was powerfully advocated by Huyghens. Newton, however, who made many interesting and important investigations in light, strongly advocated the emission theory, and the weight of his great authority turned the scale against the wave theory, in consequence of which it was in disrepute for nearly a hundred years, during which time very little progress was made in the knowledge of light.

During the life of Newton it had been established by Roemer, a Danish astronomer, that it took a certain time for light to pass from a heated body to the eye; for by calculations based on the times when the moons of the planet Jupiter were observed to be eclipsed, he had found that light traveled at the rate of 185,000 miles in a second.

Just at the beginning of the present century Thomas Young, an English scientist, brought forward new and convincing evidence of the truth of the wave theory, and showed how waves of light could be made to interfere with each other and produce darkness. This was the opening of a period of great progress. Immediately succeeding Young came Fresnel, the great French physicist, who contributed more than any one else to the development of the wave theory, and whose labors, together with those of such men as Arago and Foucault, at once brought the science of light almost to the position it occupies to-day.

But it is not true that all waves coming from a hot body are visible; even if it were not hot enough to give out waves of light it would send off waves which, though invisible, are capable of giving the sensation of warmth. These invisible waves, or heat radiations as they are sometimes called, have been made the subject of many careful investigations, and prominent among those who have devoted themselves to their study we find Professor John Tyndall, whose studies in radiant heat and diamagnetism have given him an honored place in the scientific world.

Tyndall was born in the village of Leighlin Bridge, Ireland, in 1820. His parents were poor, and this poverty brought with it the usual gifts in developing the mind and ingenuity of the little lad who was to owe all his success in life to his own individual efforts.

Like his little companions in the same condition of life, he played about the village streets, made excursions into the surrounding country, and found life a pleasant thing; for poverty to the country child brings with it none of that sordid wretchedness which so early leaves its blighting impress on the soul of the city child, to whom it comes without any grace or brightening charm.

Thus circumstanced, in spite of his parents' humble means, the boy's life passed pleasantly enough; and the lessons which nature taught him in his wanderings around Leighlin Bridge were the most useful he could have learned. He grew up a part of the beautiful world around him, and the songs of the birds, the blossoming of the flowers, and the thousand experiences of life with which he was always familiar, seemed to belong to him as much as the coloring and perfume were a part of the wild flowers he gathered.

And, besides this love and appreciation of nature, the boy was fortunate in the books which he read as a child, and which left an indelible mark on his character. His father was a man of strong religious principle, and the volumes in the family library included, with the Bible, the principal works of the most celebrated writers on theology; and, although this subject would have ordinarily no charms for a child, yet the fervid imagination, the poetic and above all the high ideality which made the duties of common life seem a religious ceremony, could not fail to make a lasting impression on the mind of a sensitive and imaginative child; while the Bible, with its wonderful imagery and powerful descriptions of nature, together with its human interest, all tinged with the deepest religious inspiration, was no less a source of fruitful teaching to the child, who read and re-read the glowing pages until he knew the volume almost by heart, and the sublime style of the Hebrew prophets had grown as familiar to him as the voice of Nature in the outdoor world.

Thus, when at seven or eight his school-days began, young Tyndall started up the hill of learning with two priceless aids—a loving intimacy with nature, and a familiarity with the grandest literature that the world has ever known.

His school days reached to his nineteenth year, during which time he pursued the usual course of study, and showed no particular talent for anything, excepting perhaps a taste for which developed itself during the last two years of his school life. He began the study of civil engineering after leaving school, intending to make it his profession, and for three years diligently studied the preparatory course, meeting with the most gratifying results.

But in 1842 he attended a course of lectures at a Mechanics' Institute, which, combined with a desire for larger study which had come to him the year before, opened wider fields of thought and gave him a deep interest in subjects unconnected with his special work.

But for five years longer he kept on in the way he had marked out for himself, completing his course of study and practicing engineering with marked success. Then, in 1847, he was appointed teacher in Queenswood College, Hampshire, and during the year that he spent in this place he became so interested in chemistry and other branches of physical science, that he determined to leave England and take a course of scientific study at some German university.

Marburg, in Hesse-Cassel, was chosen as the place of study, and here, in company with the friend whose lectures in chemistry had first interested him in natural science, Tyndall spent two years engaged in absorbing study. His student life was of the simplest kind, as money was scarce, and the end he had in view, the acquiring of knowledge for its own sake, did not point to any large remuneration from a material stand-point in the future. He studied sometimes sixteen hours a day, and although his hopes of success were sometimes overclouded by the gloomy doubts which often visit the imaginative mind, his resolve never faltered; and if his life at Marburg had borne no other fruit, it yet would have been rich in the development of that loftiness of purpose and stern devotion to duty, which at this period became such marked characteristics of the young student.

But Marburg did bring other and great prizes to him. He was under the teaching of Bunsen, the celebrated chemist, whose lectures on electro-chemistry, or the chemical changes which occur through electricity, attracted Tyndall at once, and at the same time he attended an illustrated course of lectures on radiant heat, or heat which comes in rays from the heated body, in the same manner that the heat of the sun reaches the earth. These studies were in the direct line of experimental research, and Tyndall was thus easily led to a point where he began independent investigation.

Faraday's important discovery of diamagnetism, was then attracting great attention in the scientific world. Faraday had shown that all matter could be influenced by magnetism, and had divided bodies into magnetic and diamagnetic. A bar of a magnetic substance when suspended between the poles of a magnet would point in the direction of the line joining the two poles. But if the bar were diamagnetic, it would set itself cross-wise, so that its two ends were as far away as they could get from the poles of the magnet.

But further investigation had brought to light the fact that certain substances which were diamagnetic, ceased to be so when discovered in the form of crystals. Thus, a piece of bismuth suspended between the poles of a magnet would point across the line joining the two poles, showing that bismuth was a diamagnetic substance, but a crystal of bismuth when suspended did not follow this direction, and the same was found to be true of many other substances.

In 1849 Tyndall began the study of this interesting phenomenon, and for several years carried on experiments in magnetism and electricity with the hope of arriving at some satisfactory conclusion; and, by 1855, he may be said to have reached results which were so important as to place his name foremost in the ranks of those who have studied this subject.

Crystallization, or the mysterious force by which charcoal becomes a diamond, common clay a sapphire or ruby, and by which other transformations are effected, had been an interesting subject of study from the time that science had first revealed that the same substance might exist either in the crystalline or non-crystalline state, and it was in this field of thought that Tyndall labored in his experiments on diamagnetism.

He claimed that the apparently contradictory actions of some diamagnetic substances and their crystals, were due to the structure of the substance or crystal, or the peculiar ways in which the particles forming the body were joined together. This property or peculiarity he stated was not simply characteristic of certain substances, but that, as nature acted by general laws, it would be possible, by following out the suggestions contained in this fact, to arrive at the most important discoveries in relation to the structure of the earth, and its magnetic actions; and that just as the fall of an apple suggested to Newton the theory of gravitation, so the refusal of a crystal to act in accordance with the laws that governed the uncrystallized substance might point to a law of nature which, if discovered, would unravel many of the mysteries which puzzle the scientific mind.

Tyndall also demonstrated that polarity, or the power of a substance to attract one pole of a magnetic needle and repel the other, was also a property of diamagnetic substances, with the difference that, if placed under the same magnetizing influence, a bar of diamagnetic substance would show north polarity at that end which in a bar of iron or other magnetic substance would be a south pole. It was also shown that the attractive force of magnetism is infinitely greater than diamagnetism, the magnetism of iron exceeding the diamagnetism of bismuth two and a half million times.

In 1859 Tyndall began his researches in radiant heat, a subject of great interest, not only to scientists but to all who are desirous of understanding the relations which exist between the forces of nature and the laws of life.

The power of the atmosphere to absorb the heat of the sun was then attracting attention, as it is a questions bearing directly upon human interests, as well as being a valuable subject for scientific inquiry.

The Italian physicist, Melloni, had made some very important researches in radiant heat, and had given special study to its passage through difference substances.

A body which allows heat to pass through it is said to have the property of diathermancy, just as a body which allows light to pass through it is said to have the property of transparency. And Melloni, by a series of interesting experiments, established several laws in regard to the diathermancy of different substances.

Rock salt was found to possess great diathermancy, as it allowed nearly all the heat to pass through; glass, on the contrary, which was as transparent as rock salt, was found to have little power of transmitting heat; ice and alum, equally transparent, have slight diathermancy, while clear and smoky quartz, one as transparent as glass, and the other nearly opaque, alike transmit considerable heat.

Tyndall's experiments related chiefly to the diathermancy of gases, and proved that the heat in gases and vapors was absorbed and radiated with as great differences as those which marked its passage through liquids and solids, and that it was governed by certain laws which played an important part in the distribution of heat over the world.

He found that dry air permitted heat to pass freely, but that watery vapor was possessed of great power for absorbing the heat, and this conclusion was made the basis of a most interesting hypothesis in regard to the distribution of heat over the globe.

Countries distinguished by a moist climate, like England or Ireland, were thus particularly favored, as the watery vapor, which Tyndall likened to a blanket, absorbed the heat which would otherwise have passed off by radiation from the earth, and kept a sufficient warmth to protect vegetation, just as clothing protects the human frame; and Tyndall said that if this watery vapor were removed form the air for a single summer night, the sun would rise the next day upon an island held fast in the iron grip of frost, with every plant and flower dead.

The absence of watery vapor in the atmosphere would, in like manner, account for the terrible cold of dry climates, such as Central Asia, and the nights of the Sahara desert. This theory was of special importance to geology, as it explained the origin of the glacial era; for as the earth was passing through its cooling period, the oceans, as is now the case, would naturally be warmer than the land, owing to the presence of watery vapor over their surface which served as a blanket to keep in the heat; the dry air over the land would permit the heat to pass off rapidly into space, on the contrary; and thus the rapid cooling of the land turned the mountains into receivers of the condensing vapors, which formed into the great glaciers which once covered the earth.

Another very interesting study of radiant heat, made by Professor Tyndall, related to the separation of the invisible from the visible waves or rays of light.

The fact that the light of the sun as reflected from the moon has very little heating power in proportion to its illuminating effect, had suggested to Melloni the idea of a set of experiments which resulted in the separation of heat from light on a smaller scale, and Tyndall made some successful experiments showing that the reverse was also true. In these experiments he separated the visible from the invisible rays of the sun, the lime light, and electric light, allowing the dark rays, which have the principal heating power, to pass through the intercepting medium that he used, while at the same time not a ray of light was received. With these dark rays he produced fire, melted metals, and obtained the different-colored rays of light, thus proving that the invisible rays of the sun may carry on the mightiest operations of nature, just as surely as the flower may give forth its fragrance in the darkness.

In this connection Tyndall invented a respirator for the use of firemen. This instrument, which consisted of layers of moist wool, dry wool, charcoal fragments, and caustic lime, enclosed in a wire gauze, was found to be a great protection to firemen who were unable to carry on their duties in consequence of the smoke from the burning building. The respirator effectually destroyed the bad effects of the smoke, and allowed the firemen to breathe in a room filled with the densest smoke without discomfort.

In his researches on light Tyndall also gave his attention to sound, and its relation to heat. Seamen had often been puzzled by the fact that the signals used during fogs often failed to convey the warnings in fine weather, and that the guns, gongs, and powerful whistles heard miles away during the rain could not be distinguished sometimes at short distances when the sun was shining. Tyndall suggested that this was due to the presence of invisible clouds which formed a barrier to the waves of sound, just as a dark cloud shuts out the sunshine; and, pursuing this subject later on he found that certain vapors and gases possessed the power of conveying sound in the same order as their absorption of radiant heat.

Some of the experiments leading to this conclusion related to the conversion of light into sound. Starting from the fact that thin disks of metal would produce musical sounds when struck by an intermittent beam of light, Professor Tyndall carried on a number of experiments which proved to his satisfaction that such a beam of light striking a highly absorbent vapor would even produce a more intense sound than that produced by a solid. The test experiment consisted of an arrangement by which the light struck the vapor only at intervals, the sounds being caused by the alternate expansion and contraction of the vapor, it being found that vapors and gases which allowed the heat to pass through them would produce no sounds whatever. Chloride of methyl was found to give forth sounds which, when conveyed to the ear by a rubber tube, resembled the peal of an organ in intensity.

In his pursuit of science Tyndall has added the advantages of travel, and his study on the glaciers of the Alps and the Falls of Niagara have an especial interest from the fact that they were carried on in the midst of dangers that might well have deterred a less devoted seeker after truth.

Professor Tyndall possesses a remarkable faculty for making his subjects of study understood by the unscientific mind, and his lectures in England and America have done much to make the study of science and its high objects popular, while his uncompromising love of truth, and his unimpeachable honesty in its pursuit have won him distinction from his fellow-laborers in the fields of knowledge.